In ultrasound imaging, a transducer is used to transmit ultrasound beams into the medium to be examined, for example, a region of the human body; receive the ultrasound echoes reflected from various discontinuities in the medium; and, transform the reflected ultrasound echoes into electrical signals. The electrical signals then undergo a number of processing steps and are eventually transformed into an image which can be displayed on a device such as a cathode ray tube or printed in order to be examined by a physician.
Ultrasound transducers typically consist of arrays of small rectangular piezoelectric elements. A subset of such elements used to transmit or receive an ultrasound beam is called a transmit or receive aperture, respectively. Typically, the geometrical centers of transmit and receive apertures coincide, and the ultrasound beam(s) are represented as linear beam axes originating at the center of the apertures.
The receive operation is performed by a multi-channel receive beamformer. The multi-channel receive beamformer applies delays and weights to the signals received by various receive aperture elements and sums them to obtain focused signals along the desired beam axis. The purpose of the delays is to compensate for the difference in arrival time caused by difference in propagation paths from the point of interest of the medium to the different elements of the aperture. In order to obtain ultrasound beams focused at multiple depths along the beam axis, the receive delays are varied with depth such that all the signals which are summed to obtain the echo from a point on the beam axis arrive from that same point. This is called dynamic receive focus, and the image quality is critically dependent on the accuracy of the dynamic receive delays or equivalently of the echo arrival time. It is known in the art that a delay accuracy of 1/32 Fc is desirable, where Fc is the center frequency of the transducer's frequency characteristic.
The received signals can be delayed by various methods, for example, analog and digital, but in all cases a delay, or arrival time, controller has to produce the desired delay control signals. Practical beamformers use circuits which calculate the dynamic delays in real time starting from a small number of pre-calculated initialization parameters. One such circuit is based on a computer graphics algorithm introduced in an article by Van Aken entitled, “An efficient ellipse-drawing algorithm”, IEEE Computer Graphics and Applications Magazine, vol. 4, no. 9, pp. 24-35 (September 1984), and adapted to ultrasound imaging as described in an article by Ki Jeon et al., “An efficient real-time focusing delay calculation in ultrasonic imaging systems”, Ultrasonic Imaging, 16, pp. 231-248 (1994). Variants of and improvements to this delay generation method are described in U.S. Pat. No. 5,669,384 to Park et al. entitled, “Real time digital reception focusing method and apparatus adopting the same”; U.S. Pat. No. 5,836,881 to Bae entitled “Focusing delay calculation method for real-time digital focusing and apparatus adopting the same”; “Low power delay calculation for digital beamforming in handheld ultrasound systems”, H. Feldkamper et. al., Proc. IEEE Ultrason. Symp. 2, pp. 1763-1766 (2000); “Delay generation methods with reduced memory requirements”, B. Tomov and J. Jensen, Proc. SPIE, Vol. 5035, pp. 491-500 (2003); and U.S. Pat. No. 5,724,972 to J. Petrofsky, entitled “Method and apparatus for distributed focus control with slope tracking”. In general these methods calculate the desired quantity (arrival time/delay) iteratively from one depth to the next by adding/subtracting to/from the quantity either a non-zero or a zero value depending on the sign of a decision quantity which is also calculated iteratively.
A first shortcoming of these prior art delay generation methods is their limited accuracy. The original method's error may be up to one half of the sampling period T=1/F, where F is the sampling frequency. In ultrasound imaging F is typically four times the center frequency (F=4 Fc). This results in accuracy of ⅛ Fc, 4 times worse than the desired 1/32 Fc. The prior art methods mentioned above attempt to improve on this result by combining, for example, increasing the sampling rate and increasing the algorithm complexity (and therefore the circuit complexity), both of which are undesirable as is recognized by one of ordinary skill in the art.
A second shortcoming of these prior art methods is the relatively large number of initialization parameters, that is, at least 2 parameters for each array element and each beam direction. For a typical phased array with 128 elements, 256 initialization parameters are required per beam direction.
Therefore, a method and circuit for calculating the delay or the arrival time with an accuracy of at least 1/32 Fc at a sampling rate 4 Fc, and with a reduced number of initialization parameters is required.